The present invention relates to alkali metal ferrates and, in particular, to methods for the preparation of alkali metal ferrates where the iron therein has a valence of +4 or +6.
Although the most common and familiar forms of iron in combination with other elements are those wherein iron having a valence or oxidation state of +2 or +3 is present, other compounds of iron, such as compounds of iron (IV) and iron (VI) are known in the art. In particular, ferrates, i.e., salts of iron (IV) or iron (VI) oxyanion, are known for certain heavy metals and alkali or alkaline earth metals. Iron (IV) ferrates (sometimes referred to as "perferrites") (FeO.sub.3.sup.2-) and iron (VI) ferrates (FeO.sub.4.sup.2-) are recognized oxidizing agents. Iron (VI) ferrates, in particular, are very strong oxidizing agents in aqueous solution, and stable, water soluble ferrates such as potassium or sodium ferrate are, therefore, particularly useful.
Utilization of the potentially advantageous oxidizing properties of iron (IV) and iron (VI) ferrates has been hampered by the unavailability of inexpensive, simple means for the synthesis of economical yields of sufficiently pure forms of these compounds. Methods known for production of alkali metal ferrates include electrochemical techniques wherein a 35-40% NaOH solution is used to convert scrap iron to a concentrated solution of Na.sub.2 FeO.sub.4 (H.sub.2 and O.sub.2 being by-products) using 10-15 cm.sup.2 electrodes with a 2 cm separation and an initial resistance of 2-5 ohms; the wet chemical oxidation of a soluble iron (III) compound by hypochlorite, followed by chemical precipitation of FeO.sub.4.sup.2- with potassium hydroxide to form K.sub.2 FeO.sub.4, followed by re-crystallization to obtain a high purity solid; and fusing iron filings with potassium nitrate and extracting with water.
U.S. Pat. No. 2,835,553 of Harrison, et al. discloses a multi-step process for preparing alkali metal ferrates wherein an alkali metal iron (III) ferrate (typically known as a "ferrite") is reacted at elevated temperature, in the presence of free oxygen, with an alkali metal compound (which may be the same or different than the alkali metal present in the alkali metal (III) ferrate) to produce the ferrate (IV) of the alkali metal or metals. The alkali metal ferrate (III) itself requires synthesis from more readily available materials, e.g., iron oxide. The ferrate (IV) produced in this manner may then be dissolved in water to produce ferrate (VI) according to the following equation (where, e.g., the alkali metal is sodium): EQU 3Na.sub.2 FeO.sub.3 +5H.sub.2 O.fwdarw.2Fe(OH).sub.3 +Na.sub.2 FeO.sub.4 +4NaOH
In the foregoing processes, either expensive means per se are employed (e.g., electrolysis) or complicated multistep procedures are required to produce iron (IV) or iron (VI) ferrates from more readily available materials. Moreover, in most cases, the iron (VI) ferrates produced in these methods exist in solution and require a crystallization therefrom in order to avoid the obvious handling, shipping and storage disadvantages associated with aqueous solutions.